The present invention relates to a radiation computed tomography apparatus (referred to as a radiation CT apparatus hereinbelow) and a tomographic image producing method for calculating a tomographic image of a subject from radiation projection data on the subject.
Known radiation CT apparatuses include an X-ray CT apparatus employing X-rays as radiation, for example. Known X-ray CT apparatuses include an apparatus comprising a plurality of X-ray detectors arranged in a two-dimensional manner. The plurality of X-ray detectors are disposed to have a width in a direction along a predefined axis with respect to a subject. Since rows of X-ray detectors are formed in the axis direction in the predefined width, the X-ray detectors arranged in a two-dimensional manner are generally referred to as a multi-row detector.
One known technique for reconstructing a tomographic image employing the multi-row detector is a cone BP (backprojection) process (see Patent Document 1, for example).
When using the cone BP process in the X-ray CT apparatus having the multi-row detector, the multi-row detector is rotated once around an axis as cone-shaped X-rays having a fan-shaped cross section orthogonal to the axis are emitted, for example. X-rays passing through the subject are then detected by the multi-row detector to thereby scan a region to be examined in the subject. After one scan has been completed, the multi-row detector is shifted in the axis direction at a predefined pitch, and a next scan is conducted. Such a scan technique is generally referred to as axial scanning. Since the plurality of rows of X-ray detectors lie in the axis direction, a number, corresponding to the number of rows of X-ray detectors, of tomographic images of the subject can be obtained in one scan.
Production of a tomographic image corresponding to a row of the X-ray detectors by the cone BP process uses not only detected data obtained by that row of X-ray detectors but those from X-rays passing through the region to be examined corresponding to the tomographic image and detected by the other rows of X-ray detectors. By thus increasing the amount of data for image production, the image quality of the tomographic image can be improved to some degree.
[Patent Document 1]
Japanese Patent Application Laid Open No. 2002-336239.
In the conventional cone BP process, however, the amount of detected data usable for image production is reduced in producing tomographic images corresponding to the detector rows lying at the ends in the axis direction. Thus, a need to further improve image quality exists especially for tomographic images corresponding to the detector rows at the ends.
Moreover, a tomographic image is produced using only detected data acquired in one scan around the subject's axis in the conventional cone BP process. Therefore, in an axial scan, data continuity is poor between a tomographic image acquired in a scan at one position in the axis direction and that acquired in another scan at another position. Such poor data continuity results in disadvantages such as that there is an adverse effect on the image quality of a tomographic image obtained by multiplanar reformation (MPR) using a plurality of other tomographic images orthogonal to the axis direction obtained by the cone BP process. An example of the disadvantages is insufficient smoothness in the MPR image obtained by the multiplanar reformation, in which banding artifacts occur in a portion corresponding to the border between the scans.